intermittent meromixis controls
the trophic state of warming deep
lakes
Maximilian p. Lau1,3*, Giulia Valerio2, Marco Pilotti2 & Michael Hupfer1
Vertical mixing modulates nutrient dynamics in lakes. However, surface warming reduces the range of vertical mixing and the probability of full circulation events. Important consequences of reduced vertical mixing include the sequestration of phosphorus (P) within a stagnant zone and the promotion of oligotrophication. Nevertheless, warming-induced shifts from full to partial mixing (meromixis) are not permanent and are partially reversible during exceptionally cold or windy winters. In this study, we investigated how intermittent meromixis affects lake P budgets. We examined the P cycle of a perialpine lake with variable mixing depths by pairing sedimentation and release flux measurements with sedimentary archives. We found that the amount of dissolved P surpassed that of the potentially mobile P in the sediments by a 13:1 ratio. At least 55% of the settled P was rapidly released to bottom waters isolated from flushing, illustrating the general biogeochemical mechanism that promotes deep-water P storage when lakes undergo warming. This storage process is abruptly inverted when meromixis suddenly retreats, deeper mixing introduces P pulses to the surface waters, thereby promoting phytoplankton proliferation. Our estimates showed that lakes containing up to 40% of the global freshwater volume could shift towards intermittent meromixis if the atmospheric warming trend continues. Thus, these lakes might accumulate 0–83% of their P load in irregularly circulating waters and are prone to large p pulses.
The transport of excess nutrients to surface waters is symptomatic of anthropogenic changes occurring in the biosphere1,2. Eutrophic lakes overproduce phytoplankton and their settling dead biomass depletes hypolimnetic oxygen3. Bottom water anoxia limits the habitats of all aerobic biota and modulates the biogeochemical processes determining the fate of nutrients, trace metals, and greenhouse gases4–6. Phosphorus (P) is a limiting nutrient in most freshwater systems7 and particular attention has thus been directed towards the management of urban and agricultural P export. However, the improvements in oxygen supply realised by reducing P loading might be offset by another anthropogenic stressor: increases in lakewater temperature8–10.
The global mean surface temperature has increased by 0.78 ± 0.06 °C over the last century11. There is clear evidence that lake surface temperatures have also followed a warming trend12. It has been forecasted that lake surface temperatures will increase by ≥ 70% of the anticipated increase in air temperature13. Warm lakes are at an elevated and growing risk of bottom-water anoxia because of enhanced phytoplankton growth and organic matter mineralisation14,15, increasing thermal stability, and prolongation of the stratification period16–19. Warm-ing of deep dimictic or monomictic lakes may cause them to shift to oligomictic regimes wherein the lakes no longer undergo full mixing each year6,20,21. During stagnation, the ongoing release of mineralisation products stabilises density stratification and eventually shifts these lakes towards a ‘biogenic’ meromictic regime wherein the deepest layer of the water, the monimolimnion, presents with perennial stagnation22–24.
It is expected that changes in mixing regime profoundly alter internal nutrient cycling. Protracted stagnation may lower the redox-sensitive P binding capacity25, extend the time available for mineralisation in stagnant bot-tom waters, thereby increasing the internal P supply to the euphotic zone during holomixis26. Opposed to that, partial mixing in a meromictic lake furnishes only parts of sediment-released P to the euphotic zone27 and some P remains trapped in the monimolimnion28. Warming-driven transitions towards meromixis might counter-intuitively modulate the fate of P by withdrawing some of the settled P from the seasonal cycle. Several studies
open
1Department of Chemical Analysis and Biogeochemistry, Leibniz Institute of Freshwater Ecology and Inland
Fisheries (IGB), Müggelseedamm 301, 12587 Berlin, Germany. 2DICATAM, Università Degli Studi di Brescia, Via
Branze 43, 25123 Brescia, Italy. 3Département des sciences biologiques, Université du Québèc à Montréal (UQAM),
reported that phytoplankton communities adapt to nutrient deficiencies in response to changes in deep mixing patterns29–31. These nonlinear reactions to warming that promote nutrient sequestration during changes in mixing patterns were previously predicted32–34 but the ecological consequences involved have seldom been explored27,35.
Here, we investigated the effects of a reduction in deep circulation on nutrient cycles in deep lakes. Meromixis occurs when the density stratification of a lake impedes mixing, which is itself driven by atmospheric forcing such as wind and cooling. We propose that progressive warming of the water promotes meromixis. However, the current perception of climate-driven nutrient distribution changes in lakes inadequately reflects this effect. The vulnerability of deep temperate-zone lakes to the formation and intensification of meromixis has been previously discussed21,32,33,36. Meromixis often occurs in lakes with a relative depth of > 5%37,38. Nevertheless, global warm-ing may invalidate this threshold20. The prevalence of climate-driven meromixis could increase with decreasing threshold but the effects of the former on lake nutrient balances are unknown.
An aim of the present study was to understand how a shift towards meromixis impacts lake-wide P budgets. The research was conducted on Lake Iseo which is a 256-m perialpine lake in Italy, the mixing depths of which have decreased over time39,40. We measured water column and sediment phosphorus pools, related them to their accompanying vertical fluxes (sedimentation and sediment release) during 2016/2017, and set these data within a historic context to identify the internal biogeochemical factors regulating P dynamics. Other objectives of this work included addressing the consequences of climate-driven shifts in mixing regime to lake water quality and globally estimating the general prevalence and magnitude of this phenomenon.
Results
Lake Iseo is wide and deep (A = 61 km2; z
max = 256 m). It is situated in a pre-alpine region of Italy and has a theoretical water retention time of 4.5 year. Its bathymetry is characterised by an extensive deep basin stretching along the central basin north of the island of Monte Isola (Fig. 1a). The sampling locations (A–C) were located in the central sub-basin around Monte Isola. Each one had a different water depth but all were > 90 m.
Temperature, conductivity, and oxygen profiles indicated that the water column was divided into 2–3 com-partments the vertical range of which varied annually and seasonally (Fig. 1b). In winter, a uniform temperature and conductivity zone visibly separated the deep monimolimnion characterised by a downward increase in con-ductivity and anoxia at depths > 100 m. In summer, temperature stratification separated the warmer epilimnion from the cooler hypolimnion. The former comprised a surface isothermal mixed layer and its vertical extent var-ied with wind velocity and air temperature. However, it never extended below 20 m during summer stratification. The vertical profiles of temperature, oxygen and the total and reactive phosphorus were measured at stations A–C in April and October 2017 (Fig. 2a–f). The soluble reactive P fraction (SRP) was lower than the total P (TP)
Figure 1. Map and selected characteristics of Lake Iseo. Panel (a) depicts the shoreline of Lake Iseo along with
two isodepth lines (90 m in red and 200 m in blue). Map was created by use of R software (Version 3.4.3) from published bathymetric data47,77. Labeled dots indicate sites sampled throughout the campaigns. Panels (b) show temperature (T), electrical conductivity (as 25°C, EC25) and dissolved oxygen (O2) distribution in the water column between 2016 and 2018 in the deep basin (station A). Upper and lower row of panels show profiles for the isothermic (February), and summer stratified condition (July), respectively.
near the surface. Nevertheless, both were virtually identical in deeper waters. Hence, the latter lacked biomass (phytoplankton and zooplankton) and suspended particles. Increasing proximity to the sediment coincided with a slight increase in TP at all three locations (TP range; Fig. 2e/f). Our estimates for the P dynamics of Lake Iseo were based on a fixed vertical division of the lake into 1-m horizons merged into three discrete compartments. The first was a surface layer in the range of 0–21 m. It represented an euphotic zone where SRP is assimilated during primary production. The second was a middle layer in the range of 21–91 m that usually homogenised with the surface layer in winter. The third was a deep stagnant layer in the range of 91–256 m, called "moni-molimnion", the oxygen concentration of which would only slightly increase close to its surface. Therefore, it did not substantially exchange with the surface waters during the sampling period.
Based on our measurements for April–October 2017, we calculated the pelagic TP pools for the three water column compartments and determined the cumulative TP content (Table 1). The steep TP concentration gradient and the concave bathymetry of Lake Iseo disclosed that the most extensive P pool was in the deep compartment (91–256 m; 438–476 t; Table 1) and it far exceeded the P levels in the water layers above it. The TP pool in the surface compartment was very small (~ 2%) compared to that of the deep compartment. However, the top 21 m represented ~ 15% of the total water volume of Lake Iseo (Table 1).
We measured the P fluxes between pools using automatic sedimentation traps collecting settled material over 31-day intervals. The traps estimated the TP fluxes out of the euphotic zone (surface trap at 20 m) and into the oxygen-free deep layer (bottom trap at 90 m). In the surface trap, we found material with more TP (68% of yearly flux) during the 6 months of spring and summer (March–September; Fig. 3). The shapes and integrals of
April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April April
a
October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October October Octoberb
c
d
e
f
T (°C) O2(mg L−1) P (µg L−1) winter − spring summer − autum n 8 10 12 14 16 0 4 8 12 0 50 100 150 0 m 50 m 100 m 150 m 200 m 250 m 0 m 50 m 100 m 150 m 200 m 250 mDepth
TP (and range) SRP Station A Station B Station C
Figure 2. Selected physicochemical properties of Lake Iseo’s water column. Water column profiles of
temperature (T, panels a/b), and dissolved oxygen (O2, panels c/d) were obtained from the three measurement stations A-C (colors). The symbols in panels (e/f) show concentration of total (TP, grey) and soluble reactive phosphorus (SRP, green) in the water column at Station A. The grey shaded areas show the range of TP across the three measured stations. Upper and lower row of all panels show profiles taken in April and October of 2017, respectively.
